The present invention relates to an electrostatic image developing toner and a two-component developing agent.
Heretofore, from the viewpoint of simplicity, a dry development system, using a magnetic brush, has commonly been employed as an image forming method of electrophotographic systems using an electrostatic image developing toner. Further, currently, in electrophotographic methods, development of smaller-sized higher speed color printers resulting in higher image quality has become increasingly competitive
In order to decrease the overall size of color printers, a so-called cleanerless process (refer, for example, to Patent Document 1) has been developed which makes it possible to remove a cleaner unit utilizing newly developed charging and development bias by designing a simpler and more compact transfer unit.
To overcome the aforesaid problems for apparatuses, at present, toner, having a narrow size and shape distribution, which is represented by polymerization method toners, has received increasing attention (refer, for example, to Patent Document 2).
When such a polymerization method toner is used, a decrease in the overall size of apparatuses is easily achieved. In addition, another beneficial is markedly improved image quality due to its high transferability, since the charge of the toner particles is uniform.
On the other hand, manufacturers who use external additives at a relatively large particle diameter are increasing (refer to, for example, Patent Document 3). The objective of the use of large diameter (hereinafter also referred to as large particle diameter) external additives is to stabilize development as well as to result in high transferability.
Consequently, a technique is disclosed (refer, for example, to Patent Document 4) in which a technique to enhance each toner transferability employing the aforesaid polymerization toner is combined with a technique to enhance the same employing the large diameter external additive. However, when conventional large diameter external additives known in the art are added to the polymerization toner, problems occur in which the aforesaid external additives tend to be released from toner particles due to weak adhesion to toner particles.
As one of the reasons for this release, it is assumed that polymerization toner particles are not capable of electrostatically strongly attract the external additives due to the absence of corners as well as the uniform area charge density on the particle surface.
Further, problems caused by the release of the external additives from toner particles include the following:    (a) For example, in the case of a two-component developing agent, external additives tend to be transferred to the carrier and the development rollers fitted with frictional electrification providing members, resulting in staining. Consequently, frictional electrification is hindered to tend to result in insufficient charging, whereby the working life of developing agents as well as development units deteriorates.    (b) Released external additives are stuck into the photoreceptor surface and toner particles are brought into contact with the resulting stuck portions on the photoreceptor surface to result in deposition and adhesion of the toner. Electric potential on the toner adhered portions does not decrease, whereby white spots, as called in this industry, result.    (c) Charging units are stained and tend to result in insufficient charging. As a result, halftone white streaking as called in this industry is generated.
In order to enhance the adhesion strength of external additives to toner particles, large diameter silica particles (refer, for example, to Patent Document 3) is effectively employed. However, the use of the large diameter silica particles results in problems in which the charge amount of the toner increases due to its high negative chargeability. Further, the large diameter silica particles, exhibiting high negative chargeability, assure improved transferability. However, when combined with a relatively small diameter photoreceptor, problems occur in which peeling discharge tends to occur during separation, whereby unevenness in halftone tends to occur due to discharge (in the industry, it is often called transfer repellency). These problems were particularly pronounced at low humidity.
In order to overcome the aforesaid two drawbacks, the inventors of the present invention experimented (refer to, for example, Patent Documents 5 and 6) to form a structure such that large diameter external additives are subjected to capsulation (for example, those covered with metal oxides of different compositions). However, it was not possible to sufficiently enhance the adhesion and to sufficiently overcome the transfer repellency problems.
(Patent Document 1)
Japanese Patent Publication Open to Public Inspection (herein after referred to as JP-A) No. 2002-132015
(Patent Document 2)
JP-A No. 2000-214629
(Patent Document 3)
JP-A No. 2001-66820
(Patent Document 4)
JP-A No. 2002-287410
(Patent Document 5)
JP-A No. 2002-148848
(Patent Document 6)
JP-A No. 7-89721